In recent years, along with the increase of environmental awareness, waste disposal problem of plastic products has drawn attention, and regarding shape retainers such as used for presentation packaging for various commercial products and containers such as trays for food or beverage cups, those in which various biodegradable plastic sheets are used has been developed. Among them, in particular, polylactic acid has drawn attention most as an expectable material, because its glass transition temperature is high as approximately 60° C. among biodegradable plastics and because of its transparency.
However, the glass transition temperature of the polylactic acid is low by approximately 20° C. compared to conventional materials derived from petroleum, i.e., polyethylene terephthalate, and there is a problem that heat resistance is insufficient for replacing each of present applications by the polylactic acid.
By the way, it is well known to mix 2 kinds or more of polymers with each other as a polymer blend or a polymer alloy, and it is widely applied for improving respective defects of the polymers. However, it is general that, when two kinds of polymers are mixed, the mixture is separated into respective phases in most cases, and one of the phases has nonuniform macro dispersion structure of several microns or more. In cases of such dispersion configuration, the mixture is not transparent, and also low in mechanical strength, and further, when it, is extruded in a molten and mixed state, Barus effect is likely to occur to decrease productivity in most cases. On the other hand, in very few cases, 2 polymers are uniformly mixed, which are called as compatible polymers or miscible polymers, and expected to exhibit excellent characteristics, but such cases are limited.
As methods for mixing with a resin having compatibility with polylactic acid, for example, it is known that, by mixing with polymethyl methacrylate of which glass transition temperature is approximately 100° C., glass transition temperature of the resin composition is improved (refer to Polymer, 39 (26), 6891 (1998) and Macromol. Chem. Phys., 201, 1295 (2000)). In addition, it is known that, by mixing α-hydroxycarboxylic acid polymer containing polylactic acid and poly(meth)acrylate resin, a resin excellent in hydrolysis behavior is produced (refer to JP-H8-59949 A) or that a resin composition excellent in weather resistance and mold processing behavior can be obtained by compounding acrylic compound into polylactic acid (refer to JP 2002-155207 A), but none of them discloses about an improvement of heat resistance or rigidity at high temperature.
On the other hand, in JP 2005-36054 A, it is described that a film made by mixing polylactic acid and poly(meth)acrylate-based resin and by stretching at least monoaxially is excellent in rigidity at high temperature, in JP 2005-171204 A, it is described that, compatibility is remarkably improved in case of polymethyl methacrylate of weight average molecular weight 20,000 to 300,000 and polylactic acid and only one Tg appears at center of the Tgs of the two resins, and heat resistance is improved, but any of them does not disclose at all about a technical idea in which heat resistance and biobased content, or impact resistance are compatible, and there is also no suggestion for solving the problem.
It could therefore be advantageous to provide a polylactic acid-based resin laminate sheet excellent in heat resistance, transparency and impact resistance and, furthermore, capable of obtaining a molded product of which biobased content is high, and a molded product therefrom.